Method of producing synthetic mica



Patented July 14, 1953 METHOD OF. PRODUCING SYNTHETIC MICA Jack H. "Waggoner, Newark, Ohio, assignor to Owens-CorningFiberglas Corporation, a corporation of Delaware .NoDrawing. Application May .21, .1949, Serial No. 94,723

4 Claims.

This invention relates to insulating materials having .the characteristicsofmica, and particularly to synthetic mica and themethodior pro- ,ducing the same from synthesized micaceous melts.

,Mica, as it is presently known, .is mined in the form of books which, because ,of itsmonoclinic .crystalline structure and novel crystal arrangement, may be split into thinner sheets in which iormit is generally used. ,Best use of mica for commercial consumption, whereit serves chiefly .asanelectrical insulating material, is of the relatively largesheets which arefree of impurities andother inclusions orimperfections. It is most diificult .to mine mica in1large slabs, and, therefore, the larger splittings-are very expensive. In order to make .use of the smaller sheets and splittings which arenotdiscardedbecause of impurities orjbecause of size,it isnecessary to integrate the. separate pieces with organic adhesives and then cut the bonded layer topredetermined dimension .for its intendeduse. At the very source. largequantities of theimineralare wasted and a certain proportion is discarded .in commercial here andabroad. As a result of this intense research, it has been determined that the principles of mica formation and crystallization from micaceous melts follow that of acceptable theories in physical chemistry. To secure crystals of maximum size, it is expedient tohave a minimum number of nuclei, the ideal condition calling for the formation of a :single crystal that builds up on a plantedseed. To favor formation of 'a'single or minimum-number of crystals per batch, it is desirable to cool the batch very slowly from liquidus temperature through the critical crystallizing'temperature range, which range is from about 1365 to 1.300 C. for formulations of the fiuor-phlogopite'type, More rapid crystallization .in some instances may be tolerated, where the need fora single crystal is not imperative, but in suchinstances there is a danger of forming undesirable glassy phases, which would constitute an impurity in the product. Since excess vibra tion of the supersaturated micaceous melt disturbs existing conditions and initiates the formation of more nuclei, it is desirable to carry out crystallizationwith a minimum amount of vibration.

All of these processing conditions and more, which will hereinafter be pointed out, signify the tortuous path which mica synthesis on a commercial scale must travel. To the present, the conditions thought necessary for the manufacture ,ofsynthetic mica have limited its'productionto laboratory scale, and even then daysupon days are consumed in the processing of a single batch. 1

The present invention takes the broad step of embodying the exacting conditions for mica synthesis into a commercialmass production'operaion.

It is an object of this invention .to providea method for producing mica-like products on a mass production basis.

Another object is to provide a method for rapidly and economicallyproducing mica sheets of large dimension which is particularly well adapted for industrial applications.

A further object is to provide a method for rapidly producing mica sheets having a minimum concentration of impurities, glassy phase, and substantially uniformcomposition throughout.

A still further object is .to provide a method. for producing synthetic mica sheets constituted of selected elements to give specific properties of an improved character not possible of attainment by the natural mineral.

A still further object is toprovide a continuous process for synthesizing mica-like sheets which are essentially free of aglassy phase, or other .inclusionsand still embodies the concepts of crystalgrowth developed in the prior art with a view towards producing a mica of a desired character on a mass production basis.

Recognizing the need for very slow cooling through the critical crystallizing temperature range, which is about 1365 to 1300 C. for fluorphlogopite, to produce crystals of maximum size free of glassy phase, and recognizing the need for minimum agitation whilecrystallizing to militate against .the formation of an excessive number ofnuclei, the invention embodies these concepts yet enables manufacture of synthetic mica on a mass production basis to convert the burdensome, slowand expensive laboratory techniques to -pro duction on a commercial scale Without sacrifice of physical properties.

.In practicing this invention, micaceous melts maintained above liquidus temperature are depositedin ,crystallizing molds of predetermined contour, which together are cooled down slowly through the critical temperature range with as little vibration as possible. This may be accomplished by advancing the mold with melt steadily through a heated zone which may be in the form of an elongate lehr having a temperature gradation from one end to the other calculated to permit the desired temperature drop whichmay be of about 1 C. per hour as it passes 'therethrough.

Continuous production may be achieved by the use of a number of crystallizing molds arranged in end to end relation to constitute a train of boats or flat cars dammed about the side walls to provide cavities which receive the melt toform layers of predetermined thickness. Crystallization takes place in the normal manner as the molds or flat cars are advanced through the controlled heated zone.

For example, materials constituting the batch for a micaceous melt corresponding to the nat-'- ural mineral phlogopite, but in which the four hydroxyl groups are wholly or partially replaced by flourine, may be reduced to molten condition in a platinum-rhodium crucible and held under such conditions at a temperature of about 1450 C. until homogenization is effected. The melt which then constitutes magma may be poured into molds of electrical carbon or other like material shaped in the form of flat bottomed cars initially heated to a temperature above 1365 C., which is the top end of the critical crystallizing range for the above formula. The molds containing predetermined amounts of melt or magma are then advanced with minimum vibration through an elongate heating furnace or lehr having a temperature of 1365 C. or above at one end and a temperature of 1300 C. or below at the other, with a gradual but uniform gradient in between. Instead of melting batch in a separate crucible, it is possible to lengthen the lehr to include a melting zone in advance of the crystallizing zone described thus to enable loading of the crucibles with dry batch and then to effect melting and crystallization in successive order as the crucible travels down the lehr. Of course, the melting zone will be at a higher temperature corresponding to about 100 or 200 C. above the top of the critical crystallizing range in order to melt and homogenize the batch before entering the crystallizing zone.

Assuming that temperatures of 1365 to 1300 C. exist at opposite ends of the lehr, movement of the cars or molds at a rate of one foot per hour through the lehr would call for the use of lehrs some 65 feet long. From this description, it will be clear that the exacting conditions of slow cooling with minimum vibration is achieved as the crucibles or flat cars advance upon tracks, or when suspended from conveyors, through the heating oven to produce mica-like products of a desirable character on a mass production basis. It is conceivable that crystals of a desirable character can be produced at a more rapid cooling rate, such as 3 C. per hour, in which instance a higher rate of travel or else a shorter lehr may be employed. It will be understood that other variations in conditions for crystallization may be similarly interpreted into desirable modifications of lehr construction and methods of handling.

Compared to the difliculty heretofore encountered of controlling the cooling rate of a crysta lizing micaceous melt to a steady temperature drop of 1 C. per hour or other equivalent amount, the present method, which includes maintaining constant conditions within an oven to provide a temperature gradient of a desired character therethrough, is a relatively simple matter. Thus, merely by the movement of a crystallizing crucible at a predetermined rate through the heated zone of the lehr or oven, temperature drops of a, desired character may be achieved in the crystallizing substance.

It will be equally evident that these considerations follow independent of the shape of the crystallizing crucible. For example, instead of having a flat bottomed cavity of rectangular shape, the crucible may be shaped to complex contours to produce insulating materials for specific applications or shapes for providing products such as tubes, sleevings, complex jackets, curved plates, and the like.

By the practice of the present invention, certain other advantages are secured over processes heretofore employed. It has been the practice to fuse the batch in the same crucible in which crystallization is subsequently to be effected. Since the volume occupied by the melt is approximately one-third that of the dry batch, a number of crucible loading and melting cycles are required before the crucible is filled to the desired level. Considerable time is thus consumed during which the chemical balance of the melt might be altered because of volatilization of some of the ingredients or reaction of the batch with the crucible itself. Carrying the crucible from room temperature to about 1450 C. and then cooling the crucible back to room temperature for crystallization, subjects the crucible to rigorous conditions that very few materials are able to meet. Those that do are not only expensive, but their useful life is shortened by a marked degree.

In the method embodied in the present invention, it will be apparent that melting may be carried out in a crucible constantly maintained at melt temperature. The advantages derived reside in the fact that melting will take place at an accelerated rate, and the life of the crucible will be greatly lengthened because it is not subjected to wide temperature variations.

By this invention, crucibles and molds formed of the most suitable materials may be used for their respective purposes. That is, crucibles made of electrical carbon, platinum, platinumrhodium alloys, and the like may be used for melting batch and molds formed of platinum, or electrical carbon may be used for carrying out crystallization of the melt.

In the practice of this invention, micaceous melts corresponding chemically to the fluorphlogopite modification of natural mica minerals may be used. These may be represented by the formula wherein X is usually potassium, which may be substituted in whole or in part by sodium, rubidium, cesium, calcium, barium and mixtures thereof; Y is usually aluminum or magnesium, which may be Wholly or partially replaced by varying amounts of lithium, manganese, zinc, ferrous iron, ferric iron, chromium, vanadium, tin, zirconium, titanium and corresponding materials and mixtures thereof; and Z is silicon or aluminum or mixtures thereof. 0, of course, is oxygen and F fluorine.

With materials of this type, it has been found that forsterite (aluminum orthosilicate) is formed as a primary phase as the melt passes through the first 30 to 40 C. of its critical crystallizing temperature range. To minimize the amount of forsterite formed and thereby minimize the effect forsterite might have in unbalancing the critical molecular relationship of the batch, the basic formula may be modified as hereinafter described. Variations to secure improved results and to reduce the forsterite formation range to less than 3 C. include the use of excess fluorine or fluoride in amounts up to 10 per cent by weight,

this also compensates for fluorine which might be lost by vaporization from the melt. Excess silicon dioxide in amounts ranging up to 7 to 10 per cent by weight may be employed to reduce the forsterite formation range and to increase the fluidity of the melt.

Raw materials for batch may be selected of suitable oxides, fluorides, carbonates, nitrates and silicates of the cations and. the like.

By way of illustration, but not by Way of limitation, the following formulation illustrates a suitable batch composition:

3 mols magnesium fluoride (16 per cent) 3 mols magnesium oxide per cent) 2 mols potassium aluminum silicate (69 per cent) lower end of the critical crystallizing temperature range.

This method of manufacture has an added advantage in that the lehr or heating oven may be substantially enclosed to permit maintenance of an atmosphere that minimizes volatilization of ingredients and alteration of the melt While it is crystallizing. For example, fluoride may exist in gaseous phase within the chamber.

It will be apparent from the description that there has been provided a new and improved method for producing mica products from synthesized micaceous melts on a mass production basis capable of the procurement of large sheets of mica products having lamellar characteristics capable of splitting into thinner sheets.

It will be understood that numerous changes may be made in composition, materials for handling and conditions of treatment without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In the method of manufacturing synthetic mica, the steps of advancing a fluor-phlogopite micaceous melt in suitable molds through an elongate chamber, maintaining an atmosphere of fluorine in the chamber to minimize volatilization of finer-containing compounds from the micaceous melt, heating the chamber to provide a temperature in one end portion which is above the critical crystallization temperature range for the micaceous melt and a temperature in the other end portion of the chamber which is below the critical crystallization temperature range for the micaceous melt, and advancing the crystallization molds through the chamber from the end portion having the higher temperature to the end portion having the lower temperature.

2. In the method of manufacturing synthetic mica, the steps of advancing a micaceous melt in suitable molds through an elongate enclosed chamber substantially free of circulation for changing atmospheric conditions, maintaining an atmosphere within the chamber which minimizes volatilization of ingredients from the micaceous melt in order to minimize alteration thereof during crystallization, heating the chamber to provide a temperature at least 1365 C. at the entrance end portion which is above the critical crystallization temperature range for the micaceous melt and a temperature below 1300 C. at the exit end portion which is below the critical temperature range for the micaceous melt with a uniform gradient temperature therebetween, maintaining a rate of travel of the molds through the chamber from the entrance end tothe exit end to provide for a temperature drop ranging from 1 to 3 C. per hour as they pass through the critical crystallization temperature range.

3. In the method of manufacturing synthetic mica, the step of advancing a fluor-phlogopite micaceous melt in suitable molds through an elongate chamber, maintaining an atmosphere of fluorine within the chamber to minimize volatilization of fiuor-containing compounds from the micaceous melt, heating the chamber to a temperature which includes the critical crystallization temperature range of 1365 C. in the entrance end portion and 1300 C. at the outlet end portion with a uniform temperature gradient therebetween, and maintaining the rate of travel of the molds through the chamber to provide for a temperature drop of 1 to 3 C. per hour as the molds pass through the critical crystallization temperature range.

4. In the method of manufacturing synthetic mica, the steps of advancing a micaceous composition in suitable molds through an elongate chamber, maintaining a static atmosphere within the chamber to minimize volatilization of ingredients from the micaceous composition while in molten condition, heating the chamber to provide a first heated zone at the entrance end having a temperature about -200 C. above the upper end of the critical crystallization temperature range of the micaceous composition, heating the remainder of the chamber to provide a temperature gradient ranging from the upper critical crystallization temperature for the micaceous melt in the portion of the chamber adjacent the first heated zone to below the critical crystallization temperature range at the other end portion of the chamber, maintaining a rate of travel of the molds through the chamber to reduce the micaceous composition to molten condition and homogenize the melt in the first heated zone maintained at 100-200 C. above the critical crystallization temperature and then to provide for a temperature drop ranging from 1 to 3 C. per hour as the molds pass through that portion of the chamber having a temperature gradient within the critical crystallization temperature range for the micaceous melt.

JACK H. WAGGONER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date I 710,882 Norton Oct. 7, 1902 1,814,012 Taft July 14, 1931 2,149,076 Stockbarger Feb. 28, 1939 2,185,280 Stuckardt Jan. 2, 1940 FOREIGN PATENTS Number Country Date 735,360 Germany May 13, 1943 OTHER REFERENCES Mellor, Comprehensive Treatise on Inorganic and Theoretical Chem, vol. 6, page 610. 

1. IN THE METHOD OF MANUFACTURING SYNTHETIC MICA, THE STEPS OF ADVANCING A FLUOR-PHLOGOPITE MICACEOUS MELT IN SUITABLE MOLDS THROUGH AN ELONGATE CHAMBER, MAINTAINING AN ATMOSPHERE OF FLUORINE IN THE CHAMBER TO MINIMIZE VOLATILIZATION OF FLUOR-CONTAINING COMPOUNDS FROM THE MICACEOUS MELT, HEATING THE CHAMBER TO PROVIDE A TEMPERATURE IN ONE END PORTION WHICH IS ABOVE THE CRITICAL CRYSTALLIZATION TEMPERATURE RANGE FOR THE MICACEOUS MELT AND A TEMPERATURE IN THE OTHER END PORTION OF THE CHAMBER WHICH IS BELOW THE CRITICAL CRYSTALLIZATION TEMPERATURE RANGE FOR THE MICACEOUS MELT, AND ADVANCING THE CRYSTALLIZATION MOLDS THROUGH THE CHAMBER FROM THE END PORTION HAVING THE HIGHER TEMPERATURE TO THE END PORTION HAVING THE LOWER TEMPERATURE. 